When Wallace Craig dissected the feeding behavior of doves, his experimental animal of choice, he discovered the existence of two distinct phases - an appetitive and a consummatory phase. He defined appetite as “a state of agitation”, which continues until food is presented, whereupon phase 2 begins. That’s the phase you and I call eating. It’s followed by a third phase of relative rest, which Craig called the state of satisfaction. You are forgiven if you now ask “what science nugget could possibly be hidden in this platitude.” But the best-hidden gems are often those, which are in plain sight. […]
When Craig published his paper in 1917 he described the behaviors of his doves as instinctive. In other words, being driven by some innate processes which require no conscious decision making nor any degree of intellect. Today we know a lot more about those “innate processes”, particularly that they are the result of a complex conversation between neurons and hormones playing out in the recesses of the animal brain. Not only do we know the chains of command running from brain centre to periphery we also know the hormones (at least some of them) by names, such as Neuropeptide Y (NPY) or Leptin. You don’t need to remember them. What you need to remember is that “instinctive” has matured from a black box stage to the stage of neurohormonal mechanisms, which can be tested quantitatively in the lab with experimental animals. […]
NPY is the most potent “orexigenic” peptide currently known. That’s science speak for appetite stimulating peptide. Now you also know what it means when I tell you that leptin’s effect is just the opposite, that is, anorexigenic, or appetite suppressant. Inject NPY into the right places of a rat’s brain and it will turn into a voracious eater. Give obese rats leptin, and they slim down.